One engine system being developed for controlled auto-ignition combustion operation comprises an internal combustion engine designed to operate under an Otto cycle. The engine, equipped with direct in-cylinder fuel-injection, operates in a controlled auto-ignition mode under certain engine operating conditions to achieve improved engine fuel efficiency. A spark ignition system is employed to supplement the auto-ignition combustion process during certain operating conditions. Such engines are referred to as homogeneous-charge, compression-ignition (‘HCCI’) engines.
An HCCI engine operating in HCCI combustion mode creates a charge mixture of combusted gases, air, and fuel in a combustion chamber, and auto-ignition is initiated simultaneously from many ignition sites within the charge mixture during a compression stroke, resulting in stable power output, high thermal efficiency and low emissions. The combustion is highly diluted and uniformly distributed throughout the charge mixture, resulting in low burnt gas temperature and NOx emissions substantially lower than NOx emissions of either a spark ignition engine, or a diesel engine.
HCCI has been demonstrated in two-stroke gasoline engines using conventional compression ratios. It is believed that the high proportion of burnt gases remaining from the previous cycle, i.e., the residual content, within the two-stroke engine combustion chamber is responsible for providing the high mixture temperature necessary to promote auto-ignition in a highly diluted mixture.
In four-stroke engines with traditional valve means, the residual content is low and HCCI at part load is difficult to achieve. Known methods to induce HCCI at low and part loads include: 1) intake air heating, 2) variable compression ratio, and 3) blending gasoline with ignition promoters to create a more easily ignitable mixture than gasoline. In all the above methods, the range of engine speeds and loads in which HCCI can be achieved is relatively narrow. Extended range HCCI has been demonstrated in four-stroke gasoline engines using variable valve actuation with certain valve control strategies that effect a high proportion of residual combustion products from previous combustion cycle necessary for HCCI in a highly diluted mixture. With such valve strategies, the range of engine speeds and loads in which HCCI can be achieved is greatly expanded using a conventional compression ratio. One such valve strategy includes trapping and recompression of exhaust gases by early closure of the exhaust valve during the exhaust stroke and low valve lift. Such valve control can be implemented using variable cam phasers and two-step lift cams.
Whereas the above-mentioned HCCI engine operating strategies are based upon extensive steady-state tests under fully warmed-up engine operating conditions, cold starting and engine warm-up operation pose some additional challenges related to the diversity of engine operation temperatures experience during warm-up which manifest in undesirable combustion stability and emissions.